Support casting nullable structs to non-nullable structs
Support casting nullable structs to non-nullable structs

Fixes bug 588832.

phako/vala,Distrotech/vala,spidermonkdust/vala,Distrotech/vala,Jc2k/vala,themiwi/vala,themiwi/vala,spidermonkdust/vala,spidermonkdust/vala,flobrosch/vala-fixes,rvu95/vala,playya/vala,tjol/vala-tjol,severinh/vala,kamanashisroy/aroop,phako/vala,GNOME/vala,jessevdk/vala,severinh/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,playya/vala,frida/vala,Distrotech/vala,kamanashisroy/aroop,jessevdk/vala,severinh/vala,tjol/vala-tjol,frida/vala,GNOME/vala,flobrosch/vala-fixes,flobrosch/vala-fixes,tjol/vala-tjol,themiwi/vala,jessevdk/vala,frida/vala,kamanashisroy/aroop,xdave/pvala,rvu95/vala,Jc2k/vala,playya/vala,rvu95/vala